Semiconductors are used in integrated circuits for electronic applications, including radios, televisions, cell phones, and personal computing devices. One type of well-known semiconductor devices is the semiconductor storage device, such as dynamic random access memories (DRAMs), or flash memories, both of which use charges to store information.
A more recent development in semiconductor memory devices involves spin electronics, which combines semiconductor technology and magnetic materials and devices. The spin polarization of electrons, rather than the charge of the electrons, is used to indicate the state of “1” or “0.” One such spin electronic device is a spin torque transfer (STT) magnetic tunneling junction (MTJ) device.
An MTJ device includes a free layer, a tunnel layer, and a pinned layer. The magnetization direction of the free layer can be reversed by applying a current through the tunnel layer, which causes injected polarized electrons within the free layer to exert so-called spin torques on the magnetization of the free layer. The pinned layer has a fixed magnetization direction. When current flows in the direction from the free layer to the pinned layer, electrons flow in a reverse direction, that is, from the pinned layer to the free layer. The electrons are polarized to the same magnetization direction of the pinned layer after passing the pinned layer, flow through the tunnel layer and then into and accumulate in the free layer. Eventually, the magnetization of the free layer is parallel to that of the pinned layer, and the MTJ device will be at a low resistance state. Such electron injection caused by current is referred to as a major injection.
When current flowing from the pinned layer to the free layer is applied, electrons flow in the direction from the free layer to the pinned layer. Electrons having the same polarization as the magnetization direction of the pinned layer are able to flow through the tunnel layer and into the pinned layer. Conversely, electrons with polarization different from the magnetization of the pinned layer are reflected (blocked) by the pinned layer and accumulate in the free layer. Eventually, the magnetization of the free layer becomes anti-parallel to that of the pinned layer, and the MTJ device will be at a high resistance state. Such electron injection caused by current is referred to as a minor injection.